Posttraumatic Stress Disorder (PTSD) is a prevalent condition with high rates of disability and comorbidity. Recent surveys in the greater New York area suggest that there still thousands of 9/11-related cases of PTSD due to the severe exposure to the World Trade Center Attack. Previous research has shown that both behavior therapy (prolonged exposure) and, more recently, serotonin reuptake inhibitors (SSRIs), are effective for highly chronic PTSD. However, very little is known about treatment response in individuals with PTSD due to a terrorist act of this scale. In addition, no study has specifically examined the combination of these treatments in individuals with PTSD. Controlled trials from both behavior therapy and medication studies suggest that a substantial proportion of individuals with chronic PTSD benefit from treatment but remain symptomatic and functionally impaired. There are no studies to date that systematically investigate the combination of medication treatment with behavior therapy, although this approach is widely recommended in clinical practice. This study would therefore examine the value of providing SSRI treatment simultaneously with behavior therapy. The overarching goal is the study of maximization of treatment response for adults with chronic PTSD. Participants will be 100 adults with World Trade Center Attack related PTSD. All patients will be randomly assigned to receive either 10 sessions of exposure therapy plus paroxetine, or 10 sessions of exposure therapy plus placebo. Major assessments by an independent evaluator blind to randomization status will be conducted at baseline, week 5, and week 10. Responders to treatment will be continued on pill/placebo for an additional 12 weeks with assessments at week 14, week 18, and week 22. Assessments will focus on PTSD symptoms, general functioning, interpersonal problems, and comorbid anxiety and depressive symptoms. This study possesses several strengths. It examines the combination of two cost-effective, validated, complementary treatments in a new population with chronic PTSD (victims of a large scale terrorist attack). It then examines whether benefits remain or even continue to accrue through a maintenance phase. Findings may enhance public health through the study of maximization of treatment response for this serious and common psychiatric disorder.